User blog comment:X.Quicksilver.X/The Question Blog/@comment-31815310-20170509003023/@comment-31815310-20170509004525

So heres the answer hoothoot mirror moves steelix's Fissure

onix can learn fissure in GEN1

Fissure is a move that has a 30% chance to OHKO a pokemon (at the same lvl)

Turn 1...

1. steelix use FISSURE! missed...

2. Hoothoot uses mirror move! Hoothoot uses FISSURE! IT LANDED! OHKO!

3. steelix ded